Method and apparatus for centrifugally forming of sheet thermoplastics



Aug. 31, 1948.

R. MAYNE 2,448,492 METHODS AND APPARATUS VFOR CENTRIFUGALLY FORMING 0F SHEET THERMOPLASTICS Filed March 29, 1945 2 sheets-sheet 1 Aug. 31, 1948. MAYNE 2,448,492

, R. METHODS AND APPARATUS FOR GENTRIFUGALLY FORMING OF SHEET THERMOPLASTICS Filed March 29, 1945 2 sllleelZS-Sheet 2 isfa'ctory.

Vcenter of gravity orthebubble sheet, arounda jig, from the revolving shaft.

Patented Aug. 31K, 1948 METHODANDAPPARATUS; EQR GENTRIFU- GALLY-FORMING 0F .SHEET uTHERMO- @Robert Mayne,fAkrix, Ohio, assiglnor toGoody'ear 'lv Aircraft Corporation; Akron, .1 Ohio, a corporation of Delaware applieatioriMarch 29, 1945,'sefia1N0L5sa504 This invention' relates to methods'andlapparatus for centrifugal .forming of sheetI material made of thermoplastics in general',y andinfparticular,` of methyl methacrylate, known" as Plexiglas.

"'Heretofore it was knownA toform""P1eXiglasj in sheet form into bubbles for *canopies* of Mairplanes, or for other purposesyby inserting it in heated .condition between amalel and` female forming die for giving it" the desiredshape byUA the use of pressure.- In; other cases,"'only one such die, either male or'female, was used intconjunction with air or liquid pressure, o'rthe method of free blowingthe articl'e through a jig by I air pressure was'employed. "In'using" form' dies-1r' there is always the dangerthat;` diemark-'s are left on the article, and in free blowing, the crosssectional shape of abubble is practicall'ylimited to thatfof a half circlewhich is not alwayssat- 'It' is thegeneral object of"theinven'tion to avoid and overcome the'foregoing and* other diiculties ofV and objections -to .prior-artpraetices of the provision of improved-means `and methods for forming'bubbles whichare notof# "half circular,l but' of more' *parabolic* crossr'secy"tional shape to increase the*heightftherecfras compared to bubbles 'made by thefreeblowing pIOCeSS.

Another object of the invention is toiprovidef 'relatively inexpensive; easily operated,V relatively rapid methods and apparatus for forming a' sheet "into a Ibubble by 'centrifugal forceand avoiding pressure means ofany kind.

Another object of the invention is the provi-f-A `sion of an apparatus .including a bubble'jig adapted for centrifugally forming'bubblesoi various "longitudinal and =cross'sectional`con tours.

Another object of the invention' is to" provide@ 'motor' driven apparatus which is ea'silyia'djustable so that its rotational speed'canbe regulated and the centrifugal forces varied,` either by' changing the motor speed or bychanginlgthe "draped` VAnother object of the invention is toprovide the Vapparatus with adjustments"for'longitudinally influencing the centrifugal forces towhich 'the'various bubble cross sections aresubjected.

Another object of the invention iS'to provide the apparatus with adjustments for producing articles having crossrsectionalicontours 'frombe- "tween 'half-circular" to a' jheightfconsiderably larger than `half thewidth'of the bubblebase.

The "aforesaid "objects ofthel invention'y and 'y other objects which willl become apparent asthe -description proceeds are achievedfloy providing methods ofmanufacture and an apparatusfinlcluding a-jig, overwhich afth'ermo'plastic sheet y"to belformed is draped andfclamped-thereto. lThe jig is Afastened to a` power vrdriven revolvable shaft "by two'fadjustableradial" armswhich *make it -possiblet'o change the radial distance'of' thel-jig #from the shaft at thefront` and 'rear end,` whereby the centrifugal forces actin'g'uponthe article jtobeA formed `can beregulated longitud-in'ally thereof." Accordingly;l the longitudinal Yas"vvell as the-crosssectional contours" of the article-can `l;beyvariedat 4will within wide limits; VIralijing it :possible to-'obtain streamlined bodies o'f -ah`eight 'llgreater *than-"half their base width. Bub-blesVobf tained-by this method of fabrication also yhave veryfgood` optical properties `and vare ehtirelyfree irom'mark-'oifs For a'better understanding of the inventio'nrefl erenceshould-be'had tothe' accompanyingd'ra-wings, wherein f `Figure -3 is an 'enlarged side viewofthefb'libbleA lFigure 1- lis a'side'vew of one embodiment of `the invention with the-topportion'in-cross section. Figure2 is an endview of Figure 1,

Jig, 4

'Figure 4 is a bottom plan view of Figure 3,

Figure 5 is an end' view' of Figure 3 With'one half of the `circumferential strap omitted,

' Figure 6 is a fragmentary endview` ofFigure Staken opposte'from'that oi Figure 5', and

n"Figure `|7 is a detail'showing the "grip ofthe `sheet material on the jig.

Although the'apparatus may be equipped with `a 'plurality of jigs in equilibrium -withfeach other,

the invention so far 'has' been used vonlyirrconjunction with asinglejig and has beenso illustrated and will be so described.

i With specific reference'to th'eform ofthe in- `vention illustrated inthe drawings, the numeral H-Iind-cates in general a frameworkconsistingf of 4hase channels 2 on which is mounted a pair of triangular `side supports 3 and 4 made up o`f`U- channels,` `preferably'welded together. revolvables`haft "5* provided at* one "end with `afloelt pulley B is carried in bearings 1- mounted on the supports 3 and 4. Between these bearingsV are secured tothe shaft 5 tWo cross heads 8,;spaced from each other and in each of rwhich is Vslidable,

:passing through thelshaft, a tubular arm Svproarms contain a series of holes il by "which-their effective length can be controlled and through which they are secured by a pin I2 to the cross heads 8. To the hinge joints I are attached links I3 swingable about bolts I4, whereas to the outer ends of the links is attached by bolts I4 a jig I5, designated as a Whole.

The jig I5 (Figure 3) consists of members IS of angle cross section determining the base contour of an article A to be formed of plastic sheet material into a bubble, for instance, for an airplane canopy. At its large end an arch I1 of angle cross section is welded or otherwise connected to the members I6, whereas at the small end an arch I8 of flat cross section is welded to a vertical plate I9 which in turn is united with the base members I6. A tubular transverse member 20 and -two diagonal tubular braces 2I are joined together at the top with a supporting lug 22 hinged to one of the links I3, and at the bottom with the base members I6, whereas the arch I1 ycontaining a hinge block 213 connecting to the other link II is braced against the base members by tubes 24. In addition, the base members I6 are connected by a stiifening plate 25 to prevent the spreading thereof. For securely holding the plastic sheet to be formed on the jig against centrifugal forces the frame parts I6, I1, and I8 are preferably provided with grip pins 26 which penetrate the soft plastic sheet when it is clamped thereagainst by flat loars Z'I by means of stud bolts 28 secured to the base members, and wing nuts 29, as well as by ilexible Ibands 32 and 33, respectively, provided with threaded ends 34 and held against plates 35 and 36, respectively, by wing nuts 31. In order to balance the weight of the jig, including the plastic sheet, counter weights 38, adjustable on and secured by nuts 39 to threaded rods 40 which are attached to the cross heads 8 and secured thereto by nuts 4I, are provided. The power for operating the apparatus is furnished by an electric motor 42 of variable speed fastened to the base 43 and which is operatively connected to the pulley 6 by the transmission -belting system 45, 46 and4I to provide the necessary reduction in speed of the apparatus.

Even though it is believed the operation of the apparatus will be apparent from the foregoing description, a brief review thereof will now be made for purposes of summary and simplification, and so that the steps of the forming method will be better understood.

Since the apparatus is especially advantageous for the forming of bubbles having a greater height than obtainable by the free blowing pressure process, the main shape of the bubble is determined by the jig contours, whereby the surface of the bubble which requires perfect optical properties contacts in no Way with any part of the jig during the entire manufacturing process. contour on top of the bubb-le, the jig position is adjusted radially by moving kthe front and rear arm in or out to set up the proper centrifugal forces at the various bubble cross sections when rotating the shaft. By moving the jig far out from the center of the shaft the cross-sectional contour of the sheet will assume a more parabolic and, when nearer to the center of the shaft, a more circular shape. In addition to the radial. adjustmentl of the jig the speed of the motor can be varied to expand the material more or less, whereby, of course, lthe material farthest outside tends to bulge out most. It is therefore easily possible to make necessary adjustments In order to obtain the desired streamlineA to obtain different shapes within relatively wide limits without changing the contours of the jig as such. Prior to the operation of the apparatus the weight of the jig including that of the material is carefully balanced by the counterweights. After the jig has been properly positioned and the speed, giving best results, determined, the sheet material, after being softened by temperatures of between about 250 and 350 F., is draped over the jig and fastened all around its edges by theclamping devices. In clamping, the sheet, previously provided with registering holes, is slipped over the bolts in the sides of the jig frame with the grip pins penetrating the sheet to hold it securely against slipping. The apparatus then is rotated at the proper speed to obtain the desired shape, whereby the bubble cools rapidly so that it can be marked for trimming and removed,i or it may be trimmed while on the jig to the finished dimensions. In order to avoid rapid cooling of the sheet during the process of attaching it to the jig, a hot-air filled casing in which the jig will rotate may be advantageous and into which cooling air may be admitted only after complete forming of the sheet. Such a casing may be of particular service in case multiple jigs are used which would require a longer time to be served.

It will be recognized that all objects of the invention have been achieved by not .only making it possible `to form solely by centrifugal force bubbles having a greater height than one half their base width withoutthe use of/expensive dies and without applying pneumatic or hydraulic pressure upon the sheet material, but also by varying the contours between the outer fixed edges of an article. To this must be added the flawless appearance of the finished article at a lower cost than was previously possible.

It is to be understood that the invention as shown by the example illustrated in the drawings shall not be restricted vthereto but that'different arrangement may be employed, forinstance, a plurality of jigs balanced against each other making separate balance weights unnecessary, or the revolving shaft may be arranged in vertical position, and such other changes may be made which are within the scopedened Vin the appended claims.

I claim: y

1. An vapparatus for forming by. centrifugal force bubbles and like articles made of thermoplastic sheet 'material comprising av supporting structure including a revolvable shaft and at least one pair of arms spaced from each other, means for separately lsecuring and radially adjusting each arm on said shaft, a jig having contours corresponding to the inner edge contours of the article to be formed, means for adjustably attaching the jig to each pair of said arms, means for clamping at its edges the sheet material to said jig, and means for rotating. saidshaft.

2. An apparatus for forming by .centrifugal force bubbles and like articles made of thermoplastic sheet material comprising a supporting str-ucture including a revolvabler shaft, means extendingradially from said shaft, ajig secured to the radially yextending, means, said jig having a contour corresponding to the inner edgercontours of the article to be made, means adjustably secured to the shaft for rotation therewith, and adapted to substantially counter-balance the weight of the jig and sheet material, means for clamping the sheet material at its edges to said jig,l and means for rotating said. shaft l`.toform tanaterial:wincludingrmeansion :supportingacdg zportions :of inthefmateriaitfmeansfrfor'rotatingtthe uppontine Aimeanii tolsubiectfthe sneetsmamial to.:centrifugalslforcesi kmeanstassooiatediiilithathe .soplasticf sheet :material :comprising ai supportingr 511,; supportingifmeansto vdn'dependentlyr?adjustthe :structurerincludingl :ai -revolvablesshaft: at: least ',zoneapair'. of armsI radiallytextending ffro1nsaid rshaftizand:beingoadjustably' attached sthereto at Lupointsr'spaced."axially. along;` they shaft,v aiflink I zxsponding .to2 thev inner-:edge cont-ours*l of; ther varticle to bevmadafsaidjig being.pivotallyfattached to the outer ends of said links, means for clampinglatiitsedgesithe sheet material to be formed radius of rotationf offrveriouslgspacedspcrtionsff of :fr-the:sheet:rzmaterialit-andi variable; speed means #associ-ated with.thermeansn-for?'rotatingetheisupitfportingrmeans.. i i .vhinged to each,arm,1a jighavingcontoursfcorreeil v.- 9-.'.;;Appa.ratus eforecentrifugally!fromgngasheet rmaterial, including ameansarfori supporting-@ed upportingomeansfto subjectqthe sheettmaterial itoacentrifugal';dorcesiand iaddustable :encens-fasto Said jig, .and means vfor rotating said shaft-,uasociated withitheiisupporti-ng"fmeansitonehenge ,windependentlystheferadius oforotary movement 4. In :an apparatusfdr4formirrg by centrifugal force I:bubbles and-like. articles tmade: fof; thermoplastic sheet material a jig attachedtoairotatab-le shaft, theV outer contours of said jig correagand: @thee centrifugal@ forces.;`

applied; toevarioils spaced portions Jofltheisheet i'naterial.`

wloiadli'hat fimethodoft-forming: sheets-@material sponding tofthe inneredgefcontours of the articlezoewhiele'dncludegwehe stepg 0f.snppm1ting. edg per..

to bezformed pin-like gripping means along-said ri-jig contours for cooperating-with holesiinithe \-.-:sheetmaterial to be formed, andlclamping means for securely holding ther-material only-'alongits ffriedJoy-thel jig. i

` 5..,In :an apparatus for forming byf'centrifugal Aforcegbubbles and like articlesffmade lof'l thermoplastic sheet material, a revolvable shaft, radial extensions on said shaft, a jig consisting of outer structural members with their outer contours conforming to the inner contours at the edges of the article to be formed, attachment joints on said jig connecting to said radial extensions, said attachment joints being disposed within the space surrounded by the sheet material being formed, and bracing members united with said joints and tying together said outer structural members. n

6. That method of making bubbles or like articles from thermoplastic sheet material by means of a forming jig rotated by a centrifugal apparatus including the steps, tailoring the sheet, uniformly heating the sheet to a temperature to render the sheet capable of bending `and stretching, draping the sheet at this temperature over the jig in registered position, clamping the sheet around its edge portion to the jig, selectively positioning spaced portions of the material for rotation about an axis on different radii to throw desired different centrifugal forces into the various portions of the material, and rotating the jig at requisite speed and until the shape of the sheet has assumed the desired streamlined contours and has cooled off sufficiently to hold these contours.

'7. That method of making bubbles or similar articles of thermoplastic sheet material, including the steps, of clamping the edge portions of the material in heated and soft condition to a jig conforming to the surrounding edge contours of the article to be formed and leaving the surface of the material between its clamped edge portions completely out of contact with parts of the jig, selectively positioning spaced portions of the material for rotation about an axis on different radii to throw desired different centrifugal forces into the various portions of the material, rotating the jig about an axis of rotation at a speed whereby the material is formed by centrifugal force into the desired shape, and continuing the rotation at :a speed to hold the desired shape until the article has cooled and set.

8. Apparatus for centrifugally forming sheet ...,tions..1of:ffthe material `lwhile it istin i a relatively :'pliabley-gconditiongfselcctivelwpositioriingaspaced portions of the material for rotationfi abouttan axis von. diiferenttradii to -throw,desiredy centrifcedgepoltons against said gripping mean@Car;cgyfugalvforcesintotheyarioustporticns.ontneme- .rterialrotatingathe-material.anoundttheaxis at a ing action by varying the speed" of rotation.

11. An apparatus for forming by centrifugal force bubbles and like articles made of thermo- .plastic sheet material comprising a supporting structure including a revolvable shaft, arms spaced from each other secured to and adjustable radially of said shaft, a iig having contours corresponding to the inner edge contours of the article to be formed, means for adjustably attaching said jig to said arms, means for clamping the edges of the sheet material to be formed to said jig, means for counter-balancing the weight of the arms, jig and sheet material rotating together with said shaft, and means for rotating said shaft to form the sheet by centrifugal force into the shape desired.

l2. An apparatus for forming by centrifugal force bubbles and like articles made of thermoplastic sheet material comprising a supporting structure including a revolvable shaft, means extending radially from and passing through said shaft, means for adjusting the length of radial extension on the radially extending means, jig means secured to the means extending radially from the shaft, said jig means having a contour corresponding to the inner edge contours of the article to be made, means secured to the shaft for rotation therewith and adapted to substantially counterbalance the weight of the jig means and sheet material, means for clamping the sheet material at its edges to said jig means. and means for rotating said shaft whereby the sheet is formed by centrifugal force into the shape desired.

13. That method of making bubbles or like articles from methyl methacrylate sheet material by means of a forming jig rotated by centrifugal apparatus including the steps of setting the front and rear of the jig at proper radial distance from the revolvable shaft of the apparatus, tailoring the sheet, drilling holes in the sheet for registering purposes with corresponding jig bolts, uniformly heating the sheet to a temperature of between about 250 to 350 F. in a heater, draping the sheet at this temperature over the jig in registered position, clamping the sheet around its edge portions to the iig, and

leaving the remainder of the sheet unsupported,

rotating the jig to form the unsupportedporvtions 4of the `sheet to the desired streamlined contours, and continuing the rotation at a speed to hold the sheet in the desired contour until it.y Ihas cooled to Ahold the contours.

i -14..Tna1, methodof makingbubbies or like ,articles from' thermoplastic `sheet material by means of a, forming jig rotated by-a centrifugal apparatusincluding the steps of tailoring the sheet, drilling holes in the sheet-for registering pur-poses with corresponding jig bolts, uniformly portions to the jig and leaving the centerv portion of the lsheet unsupported, rotating the jig `to move the unsupported portions of the sheet therevolvable shaft of the apparatus, tailoring the sheet, uniformly heating the vsheet to a temperature of between about 250 to 3509.1. in a heater, draping the'sheet at this temperature over the jig, clamping and supporting the sheet only at the edge of the jig and being unrestrained at any other portion of its surface, and rotating the jig at a speed to form the unsupported portion of the sheet to the desired contours, and continuing the rotation at a speed to hold the sheet in the desired contours until it has cooled off sufciently to hold these contours.

to the desired streamlined contours, and con- ,-20

tinuing the rotation at a speed to hold the sheet -in the desired contour until it has cooled to-hold the contours;

1215; That method of making bubbies or like ar- 4ticles from thermoplastic sheet material by means of a'forming jig rotated by a centrifugal appa- ROBERT MAYNE.

REFERENCES CITED'r s The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,474,432 Gimson Nov. 20, 1923 2,118,468 Jungersen May 24, 1938 2,167,539 Vienneau July 25, 1939 2,222,266 Rubissow Nov. 19, 1940 2,325,019 Rubissow July 20, 1943 2,429,692 Joyce Oct. 28, 1947 2,431,102 Ames Novfl, 1947 

